Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation. They may also take the form of implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Further, pacing systems are known which pace at multiple sites. For example, biventricular pacing paces in both ventricles and biatrial pacing paces in both atria. Hence, it is possible, that a heart may be paced in all four chambers.
A popular mode of operation for dual-chamber pacemakers is the DDD mode. Specifically, DDD systems provide atrial pacing during atrial bradycardia, ventricle pacing during ventricular bradycardia, and atrial and ventricular pacing during combined atrial and ventricular bradycardia or heart block also known as AV block. In addition, DDDR systems monitor patient activity levels for controlling pacing rate to more closely approximate the normal response of the heart to exercise, or other physiological activity demanding a faster heart rate.
Recently, pacing therapies have been advanced which encourage intrinsic ventricular activity. One such system employs an auto intrinsic conduction search (AICS) wherein the pacemaker utilizes two AV intervals. The first AV interval is a programmable base AV interval to support ventricular demand pacing. The second AV interval is an extended AV interval which may be thought of as comprising the base AV interval with an AV interval extension added to its end. An AICS is one example of an AV hysteresis algorithm. Other AV hysteresis algorithms to promote or encourage intrinsic conduction through AV interval extension have been advanced. Some AV hysteresis algorithms extend the AV interval on a periodic basis in order to search for sensed (intrinsic) R waves. The term AV hysteresis, as used throughout, shall mean any method involving AV interval extension to encourage intrinsic ventricular activity. It has been proposed to extend the PVARP interval to a duration longer than a normal (base) PVARP interval when the AV hysteresis algorithm lengthens the AV interval from the base AV interval to an extended AV interval. The term AV interval, as used throughout, shall be used to refer to an interval between a paced (A) pulse in the atrium, or a sensed P wave, and a paced (V) pulse in the ventricle.
During the extended AV interval, the atrium may have recovered on a physiologic basis to allow retrograde conduction to occur following the ventricular paced (V) or sensed (R) event and the initiation of a pacemaker mediated tachycardia (PMT). Repeated stimulation at a high rate can thereafter be sustained by heart tissue retrograde conduction combined with functional anterograde conduction that is modeled by the pacemaker when sensing the intrinsic, in this case retrograde, atrial depolarization and triggering a ventricular stimulus to be delivered at the end of the programmed AV delay. One method for preventing PMTs involves the use of programmable post-ventricular atrial refractory periods (PVARP), where the PVARP is programmed to be longer than the retrograde conduction interval.
In addition to potential PMT's, other arrhythmic heart rhythms may occur while an AV hysteresis algorithm is searching for intrinsic conduction, such as a repetitive non-reentrant ventriculo-atrial synchronous (RNRVAS) rhythm. The RNRVAS rhythm is fully described, for example, in U.S. Pat. No. 6,498,949 B2,which patent is incorporated herein in its entirety.
Examples of arrhythmic heart rhythms, that may occur when an AV hysteresis algorithm is activated, include i) AV reentrant tachycardia (AVRT) including anterograde reentrant tachycardia via the AV nodal tissue also called an orthodromic AVRT, ii) retrograde reentrant tachycardia via the AV nodal tissue called antidromic AVRT and iii) AV nodal reentrant tachycardia (AVNRT), which shall collectively be referred to hereafter as supraventricular reentrant tachycardia. Antidromic conduction is the progression of electrical activity from the atria to the ventricles through an accessory pathway with conduction back to the atrium in a backward direction from the ventricle to the atria via the AV node. Orthodromic reentrant tachycardia occurs when the electrical activity progresses from the atria to the ventricle through the AV node and returns to the atrium from the ventricle via the accessory pathway. This circular progression continues and overrides the normal conduction system. It is possible, during the extension of the AV delay in association with the AV hysteresis algorithm that retrograde conduction is allowed to occur placing an intrinsic, but retrograde, P wave that temporally coincides with an extended PVARP and is not tracked. The P wave conducts anterograde from the atrium to the ventricle with a long PR interval via either an path of slow conduction within the AV node or an accessory pathway and may then return to the atria in a retrograde manner through a second pathway, again within either the AV node or an accessory pathway, thereby allowing for sustained intrinsic supraventricular reentrant tachycardia.
Periodic extensions of the AV interval and PVARP interval may inadvertently permit retrograde conduction at the longer AV intervals in individuals who have either an accessory pathway or dual-AV nodal pathways. This is analogous to an atrial premature beat reaching the AV node when the AV node is not yet fully recovered. It occurs early in the cardiac cycle when only one of the two pathways is recovered, which may represent the pathway with slow forward conduction as it tends to have a relatively rapid recovery period. The atrial premature beat is conducted but only down the slow pathway. While the premature beat conducts down the slow pathway, the fast pathway has additional time to recover. When the forward or anterograde conduction reaches the bottom of the slow pathway within the AV node, it now finds the fast pathway fully recovered allowing it to echo back through the fast pathway. In the patient who can sustain this combination, the atrial premature beat initiates a reentrant tachycardia, in the case described within the tissue of, or surrounding, the AV node. As long as there are at least two pathways in the heart between the atrium and the ventricle with intacted anterograde conduction through the AV node via one pathway when the AV delay is extended, the impulse can echo backwards through the second pathway. The extended PVARP may place the retrograde P wave in the PVARP interval. Hence, the retrograde P wave may not be tracked (e.g. to avoid a PMT), yet based on the patient's intrinsic electrophysiology, there may still be an intrinsic supraventricular tachycardia in association with intrinsic electrophysiologic properties of the patient's heart.
A need remains for an implantable medical device that identifies and manages intrinsic reentrant tachycardia that may be initiated by the normal AV hysteresis behavior because this unmasks electrophysiologic properties of the heart that had not been previously appreciated.